1. Field of the Invention
This invention relates to impact printers, and more specifically, to maintaining the ink content on the print ribbon of such printers.
2. Related Art
The prior art of impact printing in line matrix printers is accomplished when hammers are released from retention. This causes their hammer tips to strike against an inked ribbon as it traverses between the hammers and the print media. The print media is backed-up on the other side by a hard platen, so that the impact from the hammer tip leaves ink dots on the print media. The print media can be paper, labels, multi-layer forms, including plastic and combinations of plastic and paper.
The inked print ribbon traverses at an angle between a single or dual row of hammers and the media. Each hammer strikes against the print ribbon in a dedicated zone running the length of the ribbon. The ribbon width and angle of inclination are such that the edges of the inked print ribbon are generally not struck by the hammers. This provides a boundary of tolerance to accommodate dimensional variations.
In certain line matrix printers, the ribbon reciprocates between two spools. The ribbon reverses direction when either of the spools becomes empty of ribbon.
In others, the ribbon is continuous and circulates in a loop from a cartridge across the print hammers.
In dual-row hammer line matrix printers as opposed to single row hammer line matrix printers, the arrangement is slightly more complicated. In such cases the two rows of hammers simultaneously print adjacent rows of print. This effectively doubles the throughput of the printer.
To accommodate the two rows of hammers, an inked print ribbon traverses at a slightly shallower incline across the hammers than in a single-row printer. The result is that the middle area of the ribbon is struck twice during each pass of the ribbon, while the outer boundaries are only struck once. This has adverse print quality effects. The defects in print quality when ink is depleted from a ribbon whether it be a dual-row hammerbank or a single-row hammerbank can become quite apparent.
When the ink supply in the ribbon gradually decreases it causes undesirable effects. Firstly, the density, or darkness of the printed dots decreases continuously as ink is consumed. Thus a page printed near the end of the ribbon life is much lighter than a page printed from a fresh ribbon.
Secondly, pre-inked ribbon becomes damaged as ink is consumed. This is because the ink, which lubricates the ribbon fibers, is depleted. Damaged ribbon can result in print failure at the edges of the media, as well as certain kinds of mechanical failure such as paper jams and hammer print tip clogging.
When hammer strikes are toward the center of the ribbon the unused borders of the ribbon retain a disproportionately large quantity of ink. This larger quantity of ink slowly diffuses toward the center of the ribbon. This produces darker dots on the edges of the printed page than are produced elsewhere. This effect in the art is referred to as the diffusion effect.
The fact that the middle portion of the ribbon in dual-row printers is struck twice, means that the ink is depleted more rapidly from that portion than from the edges. After a relatively small amount of printing, a light and dark pattern appears in adjacent lines of print. One of the two printed lines, for instance that which is printed by the upper row of hammers, will be darker on the right side than on the left. The next line printed by the lower row of hammers will be darker on the left and lighter on the right. In the art this is referred to as banding.
Uneven printing demands in various forms and orientations present substantial depletion of ink on a ribbon in uneven patterns. For example by printing only on the left side of the media, or by printing heavy graphics in one specific area of a page, repeatedly for many pages, can cause the print density to vary across the width of subsequent pages. This defect in the printing art is referred to as the column effect.
Another consideration is the inherent flexibility of impact printers. Such printers handle a wide range of print media. This results in a concomitant range of ink absorption rates. Consequently, ink depletion varies with print media, and location of printing on the media.
To overcome the foregoing problems, the art has developed re-inking devices. However, these re-inking devices typically only apply ink uniformly over the entire ink ribbon, while other re-inking devices generally re-apply ink to the ribbon without sensing areas in need of ink. These types of re-inking devices may not produce uniform printing when specific portions of the ribbon are used heavily or lightly in relation to the other portions of the ink ribbon.
Accordingly, it is desirable to apply ink to ribbons of impact printers that overcomes the deficiencies discussed above.
According to one aspect, the invention hereof employs a closed-loop system of ink replacement. Information about ink depletion and printing demand is used to control one or more pumps to feed the proper amount of ink back into the ribbon in areas where ink is being most rapidly depleted.
One aspect of the invention is specifically oriented to diminish the variations due to ink consumption. It helps to maintain consistency of printing or constant density of the print toward, or near the end of the ribbon life to eliminate lighter printing that is normally encountered.
Another benefit of this invention is that it diminishes ribbon damage by maintaining ink in the ribbon to lubricate the ribbon""s fibers. This helps to avoid print failure on the edges of the media as well as mechanical failure.
A further improvement of this invention is that it helps to proportionalize the quantity of ink through the ribbon to eliminate disproportionality of the ink between the edges that are not impacted and the central regions. The net result is to diminish the darker dots near the edges of a printed page. This helps to eliminate the diffusion effect.
Another aspect of the invention is to diminish the characteristics of printed material that is darker due to double strikes in certain portions of the ribbon. A concomitant of this is to lessen the differentiation between an upper row of hammers and a lower row of hammers with respect to each of the lines printed by the hammers. Thus, banding, as is known in the art, is diminished.
A further aspect is to unify the printing effect on various types of media. To this extent, the invention also serves to improve printing that takes place in concentrated areas, such as in heavy graphics and bar code orientations. This invention serves to diminish the depletion of the ink based upon such types of printing to avoid the column effect of the prior art.
The invention also provides the ability of an impact or line printer to handle various types of media that have various absorption rates.
Another consideration is that of ambient temperature conditions. This invention can compensate for changes in ambient temperature conditions by providing a multi-viscosity printer ink that can accommodate itself to a broader range of ambient temperature conditions than a single viscosity printer ink.
In summation, this invention comprises a constant density printer which maintains through the content of the ink in the ribbon, the quality of the ribbon, and a relatively proportional amount of ink in proximate location to the duty areas which are being impacted by the print hammers by way of a sensor that determines the amount of ink on the ribbon and a supply roller that is served by variable pumps to feed ink to a respective portion of the ribbon in a closed control loop.
More specifically, the invention provides for ink being pumped from a liquid ink supply into a spool or inking roller that forms a reservoir roller having a manifold. The reservoir roller supplies ink to a transfer roller which in turn deposits the ink onto the ribbon. The reservoir roller has multiple segments that can supply ink to various segments of the print ribbon. The various segments of the print ribbon have various rates of ink depletion which can be accounted for and sensed. The ink in a segment of the ribbon is replaced by the reservoir roller having a segment dedicated to a particular segment of the ribbon and replacing the ink in that segment.
A pressure roller pinches the ribbon against the transfer roller to facilitate the transfer of ink into the ribbon""s fibrous interstices and across the ribbon width by diffusion.
The ribbon inking takes place by means of appropriate amounts of ink being fed to the reservoir roller through the spool or manifold. This is controlled by a sensor which senses the amount of ink on the ribbon in multiple segments. The sensor then signals a pump to provide for a certain amount of ink to the roller in a series of applications at particular segments corresponding to segments of the ribbon.
In one embodiment, after the ribbon has been completely wound on a take-up spool, it reverses direction. As the ribbon reverses direction, it passes through the inking station after being impacted and is then wound onto the original supply spool. Thus, each segment of the ribbon encounters two printing cycles, which are alternated by re-inking cycles as the ribbon translates from one spool to the other. At the same time, the density of the amount of ink on the ribbon is sensed by the sensor, which controls the pumps to provide for an appropriate amount of ink on the ribbon at its various segments.
In another embodiment, the ribbon is continuous and passes from a cartridge across the print hammers. As the ribbon passes through the cartridge, it also passes through the inking station on a continuous basis for the appropriate supply of ink.
To further enhance this invention, the strikes of the hammers on the print ribbon in a particular location are accounted for. Thus, the duty cycle or impact cycle on a particular portion or segment of the ribbon is recorded and inking is provided in the heavily struck regions to replace any depleted ink.
A sensing of the ambient temperature conditions permits a calibration of the sensor.
A further improvement is the use of a multi-viscosity ink to compensate for changes in ambient temperature conditions.
Thus, the re-inking devices maintain the inked ribbon in an improved usable condition by keeping the ribbon""s ink quantity and distribution constant. The ink is replaced proportionally to the depletion rate and proximate to the location from which it is removed from the print ribbon, resulting in uniform printing even when certain portions of the ribbon are more heavily used than other portions.
The present invention will be more fully understood when taken in light of the following detailed description taken together with the accompanying drawings.